The present invention relates to processes and apparatus for recycling liquid carton waste material or other similar waste material which includes fibrous material, metal and plastic material.
Liquid container board, such as milk or juice carton, is typically made of layers of fibrous material, very thin metal foils, such as Al-foil, and plastic material, the plastic material usually being polyethylene. Liquid carton waste material typically contains high quality fibers, which may be recovered and re-used in e.g. production of paper roll cores or other high quality products.
It is known that the fibrous material in container board waste material can be recovered by slushing the waste material in a pulper or other suitable slushing apparatus. In the pulper waste material and water is vigorously mixed, whereby fibrous material defibrates. Defibrated fibers and water form an aqueous suspension, which may rather easily be separated from the rest of the solid waste material, the so called reject portion, mechanically, e.g. by sedimentation, centrifugation or screening. The fiber fraction in the aqueous suspension may then be further processed, e.g. cleaned, if needed, and then utilized in any suitable board or paper manufacturing process. The reject portion has until now been disposed of as a waste material, as such, without further upgrading, or it has been combusted in order to decrease the volume of the rejected waste material before transporting it to a final depot location.
It is also known to combust liquid carton waste, as such, without first separating fibrous material therefrom, in order to recover energy therefrom as thermal energy. It has further been suggested to gasify community waste or other similar waste material for providing useful product gas, which may be used for energy generation.
Liquid carton waste material, however, contains aluminum in a form which may melt already at temperatures of about 670-700xc2x0 C. Waste material, if combusted or gasified with air, should therefore be treated at temperatures well below the melting point of aluminum present therein, preferably well below 650xc2x0 C., in order to avoid problems derived from molten aluminum.
Aluminum dust may be a very hazardous component in any combustion process. Aluminum if oxidized may form locally very high temperatures, temperatures above 2000xc2x0 C., or even almost 3000xc2x0 C. Such high local temperatures in combustors can cause severe damage to the combustor. Further aluminum in dust form is an explosive material when mixed with air. Also metallic aluminum may react with alkali hydroxide, e.g. present in fly ash, and form aluminum hydroxide and hydrogen gas. It is well known that hydrogen gas under certain conditions may easily form explosive gas mixtures. Also storage of fly ash with aluminum may cause problems as formation of hydrogen gas in the fly ash may continue for a long time in its dumping place.
Liquid carton waste material also contains a considerable amount of plastic material, which today is mostly polyethylene or other plastic material, which does not contain chlorine, and which therefore can rather easily be gasified with air. Plastic material consists of almost 100% volatiles, which is possible to totally convert to gases and vapors in the air blown gasifier at temperatures of about 550-650xc2x0 C., i.e. below the melting temperature of aluminum present.
The fibrous material in liquid carton waste material, on the other hand, contains only about 75-85% volatiles. The rest of the fibrous material is fixed carbon. At temperatures between 550-650xc2x0 C., i.e. below the melting point of aluminum present, only about 60-75% of the fibrous material is converted to gases and vapors.
It may be rather difficult to control the temperature of a gasifier, so that maximum conversion of fibrous material to gases and vapors is achieved, while still avoiding problems arising from molten aluminum.
Publication WO-A-97/41269 shows a process for the recovery of aluminium and energy from used packages of the xe2x80x9cTetrabrickxe2x80x9d type and an oven for implementing such process. The oven showed has, however, a complicated structure with several moving elements. The control of temperature in different areas of the oven is probably rather difficult.
The present invention sets out to provide an improved process for recycling of liquid carton waste material and an improved apparatus therefore, which overcomes drawbacks discussed above.
It is also an object of the present invention to provide a process and apparatus for recycling liquid carton waste material in which the fibrous material present in the waste material, as well as, the heating value of the plastic material are both recovered in an optimum way.
It is a further object of the present invention to provide a process and apparatus for recycling liquid carton waste material in which also aluminum, or possible other metal present in the waste material, may be recovered in an optimum form.
It is a still further object of the present invention to provide a process and apparatus for recycling liquid carton waste material at a relatively low temperature, at which many problems arising from high temperatures in the gasification and gas cleaning processes can be avoided.
The present improved process and apparatus for recycling liquid carton waste material thereby are characterized by what is more closely stated in the characterizing portions of appending independent claims.
Thereby the present invention provides a process for recycling of liquid carton waste material or other similar waste material which includes fibrous material, metal and plastic material, including following steps
(a) introducing the waste material and a slushing liquid, typically water, into a pulper or other similar slushing apparatus for slushing of the waste material and defibration of the fibrous material therein,
(b) separating the waste material treated in step (a) into
a fiber suspension including defibrated fibrous material and
a reject portion including metal and plastic material, and
(c) cleaning the fiber suspension of step (b) and recovering the fibrous material therein for further use, in e.g. board or paper manufacturing,
(d) introducing the reject portion from step (b) into a gasifier, for gasification of plastic material therein, and
(e) separating metal, typically Al-material, from gas produced in step (d).
The reject portion may be gasified with air at a temperature below the melting temperature of the metal present in the liquid carton waste material. The gasification of a reject portion including aluminum should preferably take place at a temperature below 700xc2x0 C., typically at a temperature range of about 550-650xc2x0 C.
The gasification typically takes place in a bubbling fluidized bed gasifier. Metal foil flakes can very easily be discharged from the gasifier together with product gas generated therein. In a slow fluidized bed the metal foil material is treated rather gently and is not ground too much. Too fine, dust like, metal particles would be more difficult to separate from the product gas later on.
During slushing of the waste material in a pulper or other similar slushing apparatus alkali metal salts, such as Na- and K-salts, are dissolved in the slushing liquid. Alkali salts are thereby separated from the reject portion already at the slushing stage and will not reach the gasifier and cause problems therein.
Preferably a further step (f) comprising cooling of gas produced in step (d) in the gasifier is inserted between step (d) and step (e), for recovering heat energy from the product gas. Thereby also the separation of metal, typically aluminum, from the gas, after cooling, is made more easy.
In order to maximize the yield of fibers recovered from the waste material, the reject portion derived from the separator in step (b) may be washed in a separate additional washing step for further recovery of further fibers therefrom. Thus nearly all fibers may be recovered for further use as fibers, instead of being partly gasified.
The reject portion may be mechanically separated from the liquid fibrous suspension, by sedimentation, centrifugation, screening or any other suitable way known per se. The reject portion, from which fibers have been separated, may then be dewatered and introduced into a gasifier, for further processing. The reject portion is preferably dewatered to include  less than 50% water, e.g. 10-50% water, typically about 30% water.
Often, however, an additional cleaning of the reject portion is performed, before or after the dewatering thereof. Particularly heavy solid impurities, which may be contained in any waste material introduced into a pulper, are rather easily separated mechanically, or even if desired magnetically, from the light reject portion, consisting of light plastic material and light aluminum foil material. Heavy solid impurities typically originates from iron scraps, metal straps or similar used to bind waste material into bales. The heavy solid impurities may, of course, if desired be separated already in an earlier stage, during flushing or immediately thereafter
Most heavy impurities are, as discussed above, separated from the waste material before it is introduced into the gasifier. Some heavy metal material may, however, still remain in the reject portion when it is introduced into the gasifier. Such remaining heavy metal impurities will discharge from the gasifier together with bottom ash, being discharged from the bottom of gasifier. Impurities originating from heavy metal scrap or the like is too heavy to be entrained by the upward gas flow in the gasifier and is therefore not mixed into the product gas and the light metal material fraction being discharged from the top of the gasifier.
Light aluminum foil material typically having a thickness of only about 5-10 xcexcm, will easily be entrained by the product gas and flow out with the gas from the gasifier. The light metal fraction, thus being separated from heavy metal material possibly present in waste material, is thereafter easily separated from the product gas flow in some conventional separator, known per se. A particular separator for separating light metal flakes or foil material from gas may preferably be used.
The present invention provides a recycling process in which the light metal fraction of the waste material may be separated from both fibrous material, as well as, gasified plastic material without thereby simultaneously separating and mixing into the light metal fraction other metal material or other heavy impurities possibly present in the waste material.
The present invention provides a process according to which liquid carton waste material may be recycled such that a maximum amount of fibers present in the waste material is recovered for further useful use; metal present is recovered in a very clean form, and plastic material is recovered as a clean high heat value product gas. The heat value of gas produced from polyethylene may be in a range between 7-15 MJ/m3, typically 9-12 MJ/m3. Conventional biomaterial only provides a heat value of about 3-6 MJ/m3.